Improve type-inference in multiplying with TimesOperator

[jishnub]

This makes the following type-inferred:

julia> M = Multiplication(Fun(),Chebyshev());

julia> @inferred M * M * M
TimesOperator : Chebyshev() → Chebyshev()
 0.0   0.375  0.0    0.125   ⋅      ⋅      ⋅      ⋅      ⋅      ⋅     ⋅
 0.75  0.0    0.5    0.0    0.125   ⋅      ⋅      ⋅      ⋅      ⋅     ⋅
 0.0   0.5    0.0    0.375  0.0    0.125   ⋅      ⋅      ⋅      ⋅     ⋅
 0.25  0.0    0.375  0.0    0.375  0.0    0.125   ⋅      ⋅      ⋅     ⋅
  ⋅    0.125  0.0    0.375  0.0    0.375  0.0    0.125   ⋅      ⋅     ⋅
  ⋅     ⋅     0.125  0.0    0.375  0.0    0.375  0.0    0.125   ⋅     ⋅
  ⋅     ⋅      ⋅     0.125  0.0    0.375  0.0    0.375  0.0    0.125  ⋅
  ⋅     ⋅      ⋅      ⋅     0.125  0.0    0.375  0.0    0.375  0.0    ⋱
  ⋅     ⋅      ⋅      ⋅      ⋅     0.125  0.0    0.375  0.0    0.375  ⋱
  ⋅     ⋅      ⋅      ⋅      ⋅      ⋅     0.125  0.0    0.375  0.0    ⋱
  ⋅     ⋅      ⋅      ⋅      ⋅      ⋅      ⋅      ⋱      ⋱      ⋱     ⋱

[codecov]

Codecov Report

Patch coverage: 88.88% and project coverage change: +13.38% 🎉

Comparison is base (5bfe08f) 57.83% compared to head (d277c3a) 71.22%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #587       +/-   ##
===========================================
+ Coverage   57.83%   71.22%   +13.38%     
===========================================
  Files          80       80               
  Lines        8330     8381       +51     
===========================================
+ Hits         4818     5969     +1151     
+ Misses       3512     2412     -1100     

Files Changed	Coverage Δ
src/Operators/general/algebra.jl	88.15% <87.50%> (+4.91%)	⬆️
src/Operators/banded/Multiplication.jl	77.77% <100.00%> (+2.46%)	⬆️

... and 55 files with indirect coverage changes

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